2,460,311
60,310.5
8,766
d
19,723.5
d
0.24
cy
2,460,311
60,310.5
8,766
d
19,723.5
d
0.24
cy
The Julian Date Converter transforms any Gregorian calendar date and time into a Julian Date (JD), a continuous count of days since the beginning of the Julian Period on January 1, 4713 BC at noon UT. Astronomers worldwide rely on Julian Dates to record observations, track celestial events, and perform time-based calculations without the complexity of calendar systems.
Julian Dates are especially useful because they remove ambiguity from date arithmetic. When computing the time between two observations separated by years, months, or even centuries, subtracting two JD values immediately gives the elapsed time in days. This simplicity makes JD the universal time standard for ephemeris calculations, variable star timing, eclipse prediction, and spacecraft mission planning.
This calculator implements the standard algorithm derived from the proleptic Gregorian calendar conversion, valid for any date from 4713 BC onward. It also computes the Modified Julian Date (MJD = JD - 2400000.5), which starts at midnight rather than noon and uses smaller numbers more convenient for modern records. The J2000.0 offset measures days elapsed since January 1.5, 2000 (the standard astronomical epoch used in star catalogs and planetary theories).
The Julian Period itself spans 7980 years and was designed so that three major chronological cycles — the solar cycle of 28 years, the Metonic cycle of 19 years, and the Roman indiction of 15 years — all begin simultaneously. The current Julian Period began on January 1, 4713 BC (Julian calendar), making that date JD 0.0. Every date since then receives a unique, unambiguous number.
When entering a date, note that times are in Universal Time (UT), equivalent to UTC for most astronomical purposes. Observers at other longitudes should convert their local time to UT before entering values. The JD changes at noon UT (not midnight), so dates before noon on any given calendar day have the same integer JD as the previous day's afternoon observations.
The conversion from Gregorian calendar date (Y, M, D) to Julian Date uses the algorithm: if month is January or February, set Y = year - 1 and M = month + 12; otherwise Y = year and M = month. Compute A = floor(Y/100), B = 2 - A + floor(A/4). Then JD = floor(365.25*(Y+4716)) + floor(30.6001*(M+1)) + D + B - 1524.5, where D includes the fractional day from hours and minutes. MJD = JD - 2400000.5 and J2000 offset = JD - 2451545.0.
A Julian Date of 2451545.0 corresponds exactly to January 1, 2000 at 12:00 UT (the J2000.0 epoch). Values above this are in the future relative to J2000; values below are in the past. The MJD is convenient for modern records and begins at midnight, so the integer part of MJD changes at midnight UT. The J2000 offset is used directly in many orbital mechanics formulas to compute planetary positions.
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Results
January 1, 2000 at 12:00 UT is the J2000.0 reference epoch, giving exactly JD 2451545.0.
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Results
The summer solstice of 2024 at midnight UT. JD values are always given to at least one decimal place.
Historically, astronomers made their observations at night and needed a date that did not change during a single night's work. Starting the day at noon UT means an entire night's observations share the same integer JD, avoiding confusion at the midnight boundary.
MJD = JD - 2400000.5. It starts at midnight UT on November 17, 1858 (MJD 0). The smaller number is more convenient for satellite tracking, space mission timelines, and modern databases.
No. The Julian Date is a continuous day count invented by Joseph Scaliger in 1583. The Julian calendar is the predecessor to the Gregorian calendar, introduced by Julius Caesar. They share only the name.
The formula is exact for all dates in the proleptic Gregorian calendar (i.e., extending the Gregorian rules before 1582). For dates before October 15, 1582, the result represents the Gregorian proleptic equivalent, not what historical records would show under the Julian calendar.
J2000.0 is the standard astronomical epoch corresponding to January 1, 2000 at 12:00 TT (Terrestrial Time, nearly identical to UTC for most purposes). It is the reference point for modern star catalogs (Hipparcos, Gaia), planetary ephemerides, and coordinate systems.
Yes. Dates before January 1, 4713 BC would produce negative JD values, but the calculator is valid for any year from 4713 BC onward. Most astronomical applications only require dates from 4713 BC to the far future.
Add 0.5 to JD, then use integer and fractional parts to recover year, month, day. Numerous online tools and astronomy libraries (Astropy, ERFA) perform this inverse conversion. The algorithm is the reverse of the forward formula used here.
BJD (Barycentric Julian Date) corrects JD for the light-travel time difference between Earth and the Solar System barycenter. The correction is up to about 8 minutes (0.0056 days). BJD is used for precise variable star and exoplanet transit timing.
The constant 30.6001 ensures that integer truncation always produces the correct month increment. Using exactly 30.6 could cause off-by-one errors due to floating-point rounding near the integer boundary.
Convert local time to UTC (UT) before entering it. For example, EST is UTC-5, so 7:00 PM EST = midnight UTC = 00:00 UT the next calendar day. Astronomical observations are always recorded in UT to allow global comparison.
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